1. Field of the Invention
The present invention relates to an optical fiber amplifier which uses an optical fiber having a portion which suppresses nonlinear optical phenomena, such as four wave mixing.
2. Description of the Related Art
Optical fiber amplifiers are typically used in optical communication systems to directly amplify light signals without converting the light signals into electrical signals.
A conventional optical fiber amplifier uses an optical fiber having a core portion doped with a rare earth element, such as erbium (Er). Such an erbium doped fiber is hereafter referred to as an EDF. Excitation light is supplied to the EDF so that a light signal travelling through the EDF is amplified by an induced emission phenomena of the erbium inside the excited EDF.
Furthermore, wavelength division multiplexing (WDM) is being used to increase the transmission capacity of optical communication systems. With WDM, a plurality of different wavelength signal lights are multiplexed together into a WDM signal light. The WDM signal light is then transmitted through a single optical fiber as a transmission path.
If an optical fiber amplifier is used as a repeater for such a WDM optical communication system, it is possible to amplify the WDM signal light so that all the different wavelength signal lights in the WDM signal light are collectively amplified. The amplified WDM signal light can then be demultiplexed into the plurality of different wavelength signal lights.
Therefore, high capacity, long distance light transmission can be realized with a WDM optical communication system employing optical fiber amplifiers.
However, when conventional optical fiber amplifiers are used in a WDM optical communication system, the input optical power to the optical fiber amplifier is relatively large. As a result, there is a problem of waveform degradation due to nonlinear optical phenomena such as, for example, four-wave mixing (FWM). In general, FWM is produced if the optical power input to the optical fiber exceeds a set value. The production of FWM increases in proportion to the length of the optical fiber which transmits the large power optical signal, and becomes greater the smaller the effective optical transmission cross-sectional area of the optical fiber. Consequently, by setting an upper limit value of the input optical power in accordance with the optical fiber amplifier being used, the occurrence of four-wave mixing can be prevented.
For example, in the case where a dispersion-shifted fiber (DSF) with several tens of kilometers in length and an effective optical transmission cross-sectional area of 50 .mu.m.sup.2 is used in a WDM optical communication system, then in order to prevent the occurrence of FWM, it is necessary to set the optical power input to the DSF to equal to or less than -5 dBm.
Now, consider an optical fiber amplifier using an EDF. In general, the length of the EDF is around several tens of meters and the effective optical transmission cross-sectional area is around 13 .mu.m.sup.2. Therefore, compared to the above mentioned case of the DSF, the length of the EDF is approximately 1/1000 times that of the DSF, and the effective optical transmission cross-sectional area of the EDF is approximately 1/4 times that of the DSF. Consequently, the proportion of occurrence of FWM in the EDF compared to the case of the DSF, is approximately 1/1000 times in relation to length, and approximately four times in relation to the effective optical transmission cross-sectional area.
It is known that the upper limit value P for the input optical power to prevent the occurrence of FWM is relationship given by the following Equation (1) for a length L of the optical fiber being used, and for an effective optical transmission cross-sectional area Aeff.
Equation (1): EQU P.varies.(Aeff/L).sup.1/2
If an upper limit value for the input optical power for the case of an EDF is estimated using the above relationship of Equation (1), then this is approximately 12 dB higher than the upper limit value for the case of the DSF. Hence, the upper limit value for the EDF can be considered to be around +7 dBm.
However, in the case where an optical fiber amplifier using an EDF is used as a repeater for a WDM optical communication system, there is a possibility that the power of the WDM signal light amplified inside the EDF exceeds the above mentioned upper limit value.
For example, FIG. 1 is a diagram showing the optical power inside an EDF of an optical fiber amplifier, from an input end to an output end of the EDF. As shown in FIG. 1, inside the EDF, the WDM signal light input to one end of the EDF increases in optical power with propagation in the longitudinal direction of the fiber. Therefore, even if the optical power at the input end is equal to or less than the upper limit value, there can be the situation where the optical power exceeds the upper limit value before reaching the output end. Consequently, during the interval from where the optical power exceeds the upper limit value until the signal light output end, the WDM signal light is propagated while being amplified. Hence, there is the likelihood that FWM will occur, thereby degrading the waveform of the WDM signal light.
As can be determined from Equation (1), the occurrence of FWM in the EDF can be suppressed by shortening the length of the EDF. However, in order to shorten the length of the EDF while maintaining the same gain, the absorption and the radiation per unit length of the EDF with respect to the signal light must be increased. For example, as a technique for realizing this, use of an EDF with a high erbium concentration, or use of an EDF with a large erbium doped diameter has been considered.
However, if the erbium concentration is increased, energy conversion between the erbium ions occurs, resulting in a density extinction which lowers the excitation efficiency of the EDF. With current EDFs, the standard for the erbium concentration is around 500 ppm. Furthermore, if the erbium doped diameter is increased, then the erbium is doped in a portion where the energy density of the excitation light is low and the excitation effect is minimal. Hence, as with the case where the erbium concentration is increased, there is the likelihood of a drop in the excitation efficiency of the EDF.
Therefore, the shortening of the length of the EDF in order to suppress the occurrence of FWM, and modification so that the amplification characteristics of the EDF are not degraded, are contrary to each other, and it is difficult to realize both simultaneously.